Thermal lithographic plates are plates that are exposed or imaged by infrared radiation and/or heat. With certain plates the infrared radiation may initiate a photochemical reaction in, for example, an onium compound, present in a coating on the plates. In such embodiments an infrared dye, also present in the coating, acts as a photosensitiser, absorbing the infrared radiation and sensitising the decomposition of the onium compound. With certain other plates, heat is itself the direct cause of imaging and is not believed to induce either a chemical decomposition of components within the coating or a photochemical reaction. In such embodiments, heat may be delivered to plates in a platesetter by a number of methods including, for example, contacting the plates with a heated body, or using charged particles or electromagnetic radiation that can be absorbed in a coating on the plate to thereby generate heat. One method of imaging thermal plates employs infrared radiation via an IR laser.
So-called Computer-To-Plate (“CTP”) technology has in part been responsible for a move towards thermal lithographic plates. The required pattern in the coating on the lithographic plate may be “written” by an infrared laser, under digital control.
Two principal classes of thermal plates are now commercially available. Positive working thermal plates, such as the ELECTRA plate of Kodak Polychrome Graphics, may be exposed by imagewise heating of regions of the plate coating, typically by exposure to imaging infrared radiation, rendering to these regions more soluble in a developer and, thereby, more readily removed by a developer. This class of plates is disclosed in, for example, the PCT patent application published under the number WO 97/39894.
Negative working thermal plates, on the other hand, may be exposed to imaging infrared radiation as previously described for positive plates. They are then subjected to an overall heating step after imaging but before development. This is typically referred to as a “preheat step.” This heating step is believed to selectively crosslink those regions of the coating that were selectively imaged, rendering them preferentially less soluble in a developer. Thus, on development or processing of such a plate, the regions that were not imaged are selectively developed away. A typical example of such a plate is the THERMAL PRINTING PLATE/830 of Kodak Polychrome Graphics. The technology typically utilised in these plates is disclosed in, for example, U.S. Pat. Nos. 5,340,699, 5,372,907 and 5,491,046, and can involve the use of an imaging layer comprising a resole resin, a Novolak resin, a latent Bronsted acid and an infrared absorber. The contents of these patents are incorporated herein by reference.
These negative-working plates are commonly imagewise exposed by near infrared laser light at 780-1400 nm, then pre-heated in an oven to about between 100 and 200° C. for between about one and three minutes before processing through a processor that sequentially provides the following:                1. an alkaline developer bath;        2. a rinse section; and        3. a gumming section;which can be in an integrated processor. The plate can then be mounted on a lithographic press, whereupon many tens of thousands of prints may be obtained, although this is typically limited to 250,000 copies before the plate image is worn out. If extra copies are desired from the plate, it is known to transport the plate through a forced air oven heat the plate further, following the processing step, typically at about 285° C. for about 2 minutes. This process, called “post baking,” can extend the life of the plate on the press to over 1,000,000 copies.        
However, this baking process suffers from significant disadvantages in that it uses a very large oven that consumes a significant amount of electrical power with the attendant cost implications. A further disadvantage is that it has resulted in excessive or uneven heating, which can make the baked plate wavy and difficult to accurately mount on to the press. A third difficulty is that a protective gum is applied to the plate before baking, according to a widely known process sometimes referred to as the Thermotect® process, to prevent background contamination occurring during the post-baking treatment. This gum is then removed and a normal plate finisher applied if the plate is to be stored before printing, thereby requiring further time and labour to be expended. Indeed, it is generally also necessary to remove plate finisher applied following the development process before applying the protective gum prior to baking, thereby introducing even further delays and inefficiencies into the process. Notwithstanding these problems, however, plates of this type are generally baked in order to achieve greater run lengths on the press, thereby facilitating production of the desired number of copies at the end of the print run.
Consequently, it would be significantly advantageous if a treatment could be devised that overcame the disadvantages associated with post-baking methods whilst still allowing for some or all of their significant benefits, including increased run lengths, to be achieved.